MELBOURNE, Aug 9 (Reuters) - Private firm Danusa Tambang Nusantara has agreed to buy the Martabe gold mine in Indonesia from EMR Capital and its partners for $1.21 billion, including assumed debt, EMR said on Thursday.

Private equity firm EMR and its partners Farallon Capital and two Indonesian investors bought the mine for an enterprise value of $775 million in 2015 and have since doubled its reserves and cut its production costs, EMR Chief Executive Jason Chang said.